1. Field of the Invention
The present invention relates to a new aqueous coating composition containing a cationically modified polyol component dissolved and/or dispersed in water, optionally a reactive diluent and, emulsified therein, a polyisocyanate component; a process for the production of this coating composition; and its use for the production of coatings on water-resistant substrates.
2. Description of the Prior Art
Aqueous coating compositions are gaining increasing importance for economic and ecological reasons. However, the replacement of conventional, solvent-based coating compositions is proceeding more slowly than expected.
There are numerous reasons for this. Aqueous dispersions frequently still have processing disadvantages when compared to coating compositions dissolved in organic media. With aqueous solutions there is also the conflict between providing sufficient water dispersibility or solubility versus the contrary effect resulting therefrom of the lower resistance of the coatings to water. This is not a problem with coating compositions dissolved in organic solvents. In addition, there are also processing problems in this regard which result from the high viscosity of the aqueous coating compositions. These problems have previously been overcome by the use of organic auxiliary solvents. However, the amount of auxiliary solvent used in this connection is limited, since, otherwise, the ecological impact of the aqueous systems is lessened.
For this reason, in binder systems crosslinked with melamine resins (U.S. Pat. Nos. 4,031,052, 4,171,294 and 4,276,210 and German Offenlegungsschriften 2,446,760 and 2,847,532), water-dilutable reactive diluents have previously been used. These resins have a favorable effect on the solubility properties of the polymer systems and are also incorporated in the coatings by melamine resin crosslinking. However, the reactivity of many aqueous melamine resins is so low that high crosslinking temperatures are required such that the reactive diluents can escape from the coatings before crosslinking occurs.
Only recently have aqueous two-component polyurethane systems become known. For example, German Offenlegungsschrift 3,829,587 discloses systems in which the binder is based on a polyacrylate resin dissolved or dispersed in water in combination with a polyisocyanate containing free isocyanate groups emulsified in this dispersion or solution. The systems are essentially solvent-free, as is evident from the fact that the solvents used in the production of these polymer resins are removed before preparation of the aqueous composition. U.S. Pat. No. 5,194,487 discloses aqueous two-component polyurethane systems based on an anionically modified isocyanate-reactive polyurethane and a water-dispersible polyisocyanate. These known prior art systems can be used to produce high-grade coatings that are comparable in their properties to coatings prepared from solvent-containing coating compositions of analogous structure.
It has now surprisingly been found that aqueous two-component polyurethane coating compositions in which the polyol component is cationically as opposed to anionically modified have a considerably longer pot life and are as suitable as analogous systems based on anionically modified polyhydroxyl compounds for the production of high-grade coatings. The coating compositions according to the invention, which are described in more detail below, have pot lives of at least 8 hours to as much as several days.